1. Field of the Invention
The invention relates to a ceramic heater well-suited for a ceramic glow plug incorporated into a diesel engine.
2. Description of Prior Art
In a prior art ceramic heater disclosed by provisionally published Japanese Patent Application No. 6-251862, there is provided a ceramic body sintered with silicon nitride as a main material. MoSi.sub.2 particles (mean granular diameter: 3.about.25 .mu.m), a thermal coefficient of expansion of which is greater than that of the main material, are dispersed in the main material to contain 3.about.25% by weight. This ameliorates the oxidation resistant property so as to improve the endurance ability which is represented by duration until a heater resistor is disconnected to cease the heat generation due to repetitive heating and cooling cycles.
Upon checking the performance of the ceramic heater, the ceramic heater has been mounted on a glow plug to carry out an endurance experimental test by repeatedly energizing and deenergizing the ceramic heater. As a result, it has been found that cracks may occur on the silicon nitride-based ceramic body even though the heater resistor is not disconnected.
The reason why the cracks occur is considered as follows:
Due to the greater thermal coefficient of expansion of MoSi.sub.2 compared to that of the silicon nitride-based ceramic body surrounding the MoSi.sub.2 particles, thermal stress occurs when subjecting to the sintering process and the repetitive heating and cooling cycles. Depending on the granular diameter of the MoSi.sub.2 particles, the thermal stress often increases more than the silicon nitride-based ceramic body can endure in the context of physical strength, as exemplified by a glow plug which is rapidly heated and cooled repeatedly during operation.
Therefore, it is a main object of the invention to provide a ceramic heater which is capable of enduring the repetitive energization and deenergization for an extended period of time without inviting cracks on the silicon nitride-based ceramic body.